Transmitting and receiving apparatus and method for electromagnetic prospecting



Dec. 1, 1953 w. M. BARRET 2,651,466

TRANSMITTING AND RECEIVING APPARATUS AND METHOD FOR ELECTROMAGNETIC PROSPECTING Original Filed April 19, 1943 6 Sheets-Sheet 1 lWl/fam M fiarre/ Dec. 1, 1953 w. M. BARRET 2,651,466

TRANSMITTING AND RECEIVING APPARATUS AND METHOD FOR ELECTROMAGNETIC PROSPECTING Original Filed April 19, 1943 6 Sheets-Sheet 2 Zmnentor W/Y/lam M. .Barra/ W anomeu w. M. BARRET 2,661,466 TRANSMI'ITIIIG AND RECEIVING APPARATUS AND METHOD FOR ELECTROMAGNETIC PROSPECTING Original Filed April 19. 1943 6 Sheets-Sheet 3/ Dec. l, 1953 FIG. 4

Dec. 1, 1953 RRET 2,661,466

W. M. BA TRANSMITTING AND RECEIVING APPARATUS AND METHOD FOR ELECTROMAGNETIC PROSPECTING Original Filed April 19. 1943 6 Sheets-Sheet 4 &

Zmventor Il /703m M flax-ref Dec. 1, 1953 w. M. BARRET 2,661,466

TRANSMITTING AND RECEIVING APPARATUS AND 6 METHOD FOR ELECTROMAGNETIC PROSPECTING Original Filed April'19. 1945 6 Sheets-Sheet 5 3110mm Will/.2917) M San-e2 W. M. BARRET TRANSMITTING AND RECEIVING APPARATUS AND Dec. 1, 1953 6 Sheets-Sheet 6 Zhwentor W/Y/iam M. Barref Gttor eg Patented em 1, 9

()riginal application April 19, 1943, Serial No.

Divided and this application March 26, 1945, Serial No. 584,960

9 Claims.

This invention relates to the art of electrical prospecting, and more particularly to an improved system for introducing electromagnetic energy into the earth and receiving it therefrom, the said system being especially adapted for use with the radio methods of prospecting.

The present application is a division of applicants co-pending parent application subsequently abandoned for Transmitting and Receiving Apparatus and Method for Electromagnetic Prospecting, Serial No. 483,638 filed April 19, 194.3.

The transmitting and receiving antennas employed with the conventional radio methods of subsurface exploration have customarily followed the well-known types used for radio communication between widely spaced points on the earths surface. It is recognized that in the communication art it is desirable that the electromagnetic radiation be propagated principally through the air, but that for radio prospecting it is an essential requirement that the radiation be transmitted into the earth and subsequently received therefrom. The problems and requirements of the two applications are therefore entirely different, and it is evident that the same type of radiating and receiving means would not be adapted to both communication work and radio prospecting.

The herein invention discloses transmitting and receiving means for electrical prospecting which overcome the defects inherent in the antenna systems formerly utilized.

One of the objects of the invention is, accordingly, to provide a novel and useful means for transmitting electromagnetic energy into the earth.

Another object of the invention is to furnish an efiective means for receiving electromagnetic energy from the earth.

Another object is to make available an efiicient transmitting means and receiving means for introducing electromagnetic energy into the earth and receiving it therefrom, whereby the said transmitting means and receiving means may be employed collectively to practice various radioprospecting methods, or. they may be used individually with conventional systems for radio exploration.

Another object is to furnish a method of effectively introducing electromagnetic energy into the earth, and receiving it therefrom.

An additional object is to supply a method of utilizing the effective introduction of electromagnetic energy into the earth and its reception therefrom, whereby the said method may be practiced in accordance with the disclosures in the prior art.

A further object is to provide a method for the directive transmission of electromagnetic energy into the earth in any desired direction, and for the directive reception of the said energy from any desired direction.

Other objects of the invention will be apparent from a consideration of the description which follows, and the drawings appended thereto, wherein:

Fig. 1 illustrates diagrammatically the radiation from a vertical quarter-wave antenna whose base is positioned at an interface between air and earth.

Fig. 2 shows a radio transmitter that is inductively connected to a circuit which includes an energizer placed substantially in an air-earth interface, the radiation from the said energizer impinging on the earth at approximately grazing incidence and being refracted therein.

Fig. 3 is a schematic diagram displaying the reflection of electromagnetic Waves at a subsurface reflector lying parallel to the air-earth interface, when the said waves enter the earth at substantially grazing incidence.

Fig, 4 is a schematic diagram displaying the reflection of electromagnetic waves at a subsurface reflector that is inclined downward toward the wave source, when the said waves enter the earth at substantially grazing incidence.

Fig. 5 is a schematic diagram displaying the reflection of electromagnetic waves at a subsurface reflector that is inclined upward toward the wave source, when the said waves enter the earth at substantially grazing incidence.

Fig. 6 shows the input circuit of a radio receiver that is excited by a pickup lying substantially in an air-earth interface.

Fig. 7 illustrates a radio transmitter that is inductively connected to a circuit which includes two energizers placed substantially in an airearth interface, or alternatively, below the said interface.

Fig. 8 illustrates the input circuit of a radio receiver that is excited by two pickups lying substantially in an air-earth interface, or alternatively, below the said interface.

Fig, 9 is a diagram that exhibits the bidirectional horizontal radiation pattern of a quarterwave insulated energizer placed flat on the earths surrace.

Fig. 10 is a diagram that exhibits the substantially unidirectional horizontal radiation face and excited with currents having particular:

phase and amplitude relations.

Fig. 12 illustrates diagrammatically an arrangement of apparatus which is'su'itable for driving the two-element array of Fig. 10.

Fig. 13 illustrates diagrammatically an ar-' rangement of apparatus which is suitable for driving. the sir-element arraynof Fig. 11.-

Fig. 14 illustrates the circuit of applicants invention connected with a transmitter anduti-J."

lizing two collinear quarter-wave insulated energizers placed below the air-earth interface?" and Fig. 15 illustrates the applicationof thein vention to a receiver using two collinear quarterrwave insulated energizers placed below the. airearth interface.

It has been the generally accepted belief that the..-.radiation from an antenna is directly pros portional to its height above the earths surface, and radiation equationsin good textbooks, for example: J. H. Morecroft, Principles of Radio Communication, p. 883, John Wiley and Sons, Inc., lew York (1933), indicatethat no radiation would be obtained from an antenna having Zero height. For thisreason, radio-prospecting systems have in the past made use of some form of transmitting antenna which was supported at an appreciable height above the earth.

Atype of antenna not uncommon in radio prospecting is the quarter-Wave vertical radiator i (Fig. 1), whose current distributionis indicated by the dashed curve 2. The field intensity ata distant point due to this antenna is'the summation of the effect of each radiating element, such aselement 3, which propagates a ray l direct. to

the said distant point, and a rayE' which reaches the. distant point after reflection from the earth 5; If the earth 6 were perfectly conducting; then-- all of the energy in the ray 5 would be reflected 8, where it is refracted according to Snells Law in the direction of the arrow 9.

It is, of course, the refracted energy-whichenters the earth that is efiective in the operation ofradio-prospecting methods, and'since the reconductor of the proper form, and that it may be located either on the surface of the earth or buried therein.

The fact that a current-carrying element having substantiallyzero (or negative) height --abovc the earth will propagate electromagnetic waves is a contradiction of the radiation equations re- 'ferred to previously. The disagreement may be explained, however, by remembering that the said radiation-equations are postulated on the propa- 'gation of energy through the air above the earth,

and that the. said current-carrying element, lo cated substantially in or below the air-earth interface; resultsinan energization of the earth volume adjacent thereto, and the coneequenttrans mission therefrom of electromagnetic energy through the earth;

In View of the particular function performed by the-said current-carrying element positioned as here described, it shall henceforth in this specification be called at times an energizerf? Av number of. important difierences distinguish.

the said energizer from the conventional air antennas used in communication work andin radiov prospecting. Attention already has been directed to one of the chief differences, that is, its location immediately adjacent to or within the earth, Whereas the eiiective operation of conventional antennas requires that they be placed in the air, a considerable distance above the earth, and that their radiating portions be insulated carefully from the earth. Another distinction that has been pointed out, andone which is significant in radio prospecting, thatthe said energizer concentrates in the earth the energyit transmits, While on the contrary the ordinary antennas concentrate their radiation in the air. The said energizer also exhibits directional characteristics that are vastly different from those of air antennas of similar iorm and proportions; for ex ample: a half-wave horizontal antenna located in the air, and Well removed from the earth, will radiate bidirectionally at right angles to its length, but a half-wave horizontal energizer, placed flat on the earths surface, will fire bidirectionally-along its'length. Moreover,- the half-wave' air antenna will radiatef crizontally polarizedwaves, while the energizer. propagates vertically polarized waves. Still'another difference is that the electrical proportions of the energizer must be predicated on the wave length of the-transmitted Waves in the earth, rather than on the length of the waves in free space, as is the case-for antennas operating in the air, and well above the earth. This last difference frequently fracted component may constitute 'but-avery small part of the energy radiated by the antenna I, it is evident that this form of radiating system is inefiicient and poorly adapted to'the require ments of radio prospecting. i-"his criticismapplies not only to the quarter-wave vertical an tenna shown in Fig. 1, but to all of the radiators customarily employed in prospecting work.-

Careful research has developed the fact that-a current-carrying element placed substantially in or below the air-earth interface is suitable for propagating electromagnetic Waves for themoderate distances requiredin radio, prospecting, and

ingelement may-be either an insulated or abare results in a great reduction in size for the energizer, as under some conditions the'wave length in-theearth may be less than one-fourth that in free space.

It is evident, therefore, that a conventional f r propagating electromagnetic a" es into the etics are altered Jcribe'd heron.-.

earth until its essential charact toconform to the energizer de.

One embodiment of the invention is illustratedin Fig. 2, wherethe insulated metallic conductor cm, which 'is held by the insulators i i and i 2 and insulated from the. earth in by the covering it,- is connected to-the variable condenser 54, thence through the inductor. l5 andcurrent-indicating meter I 6 to the metallic'ground-rod l i, r

If the 4 conductor 1 8 beet the preper length-and if-the inductor be excited by the radio transmitter T, for example, by inductive coupling with the coil 18, then the circuit which includes the conductor 50 may be made to resonate at the frequency of the transmitters oscillations by adjusting the variable condenser i for maximum deflection of the meter it.

When the said circuit is brought to resonance, the conductor it acts as a radiator of electromagnetic waves, and the radiations therefrom will be transmitted to some extent in practically all directions from the said conductor, but chiefly along its axis.

As the conductor ii], hereinafter termed the energizer it, lies substantially in the air-earth interface it, there is no appreciable reflection of the radiation at the said interface, and the energy is transmitted freely into the earth 6. However, as stated previously, most of the propagated energy is concentrated substantially along the axis of the said energizer, in the directicns of the ar rows 2b and 2|, and it is this concentration of energy which serves the most useful purpose in the operation of the energizer illustrated.

Consider the arrow it, which represents a bundle of electromagnetic waves moving to the right of Fig. 2. These waves impinge on the airearth interface 19 at approximately grazing incidence, and hence (according to Esnells Law) are refracted into the earth at an angie r=sinl/n, where n is the index of refraction of the earth material with respect to air. One of the refracted waves is represented by the line 22, and as the bundle of waves progresses in the direction of the arrow 20, additional waves peel off and enter the earth at grazing incidence (angle a: 98 degrees), as indicated by the lines 23 and 22.

In view of the fact that a large part of the energy fired by the energizer it strikes the airearth interface H! at angles approximating grazing incidence, and because practically all such energy finally is refracted into the earth t, it evident that the system under discussion is a highly effective means for transmitting into the said earth a large proportion of the total energy propagated by the said energizer.

The preferred means for practicing the invention disclosed in Fig. 2 employs for the energizer it an insulated copper wire, for instance, a rubber-covered single-conductor stranded cab1e,.

whose length is approximately one-quarter the wave length (in earth) of the radiation used, and which is covered by a rubber tube having a wall thickness of about inch. The said energizer is fastened to the low-loss insulator II; passes through the similar insulator I2, and thence to the tuning components indicated, which are enclosed by the electromagnetic shield 25. The tube It, which preferably is made substantially waterproof, is in ordinary practice placed flat on the earths surface I9, and oriented to obtain maximum transmission in the desired direction.

It will be obvious to those versed in the radio art that various modifications may be made in the preferred transmitting means here described without departing from the broad principle of the invention. For example, the energizer It! may be a bare conductor of various sizes and shapes instead of the preferred insulated Wire; the covering [3 may be dispensed with if the insulation on the said energizer is adequate, and in some instances it will be found satisfactory to use a bare conductor in intimate electrical contact with the earth 6; the said energizer may have various lengths, but preferably approximating quarter-wave-length multiples of the propagated Waves, it being understood that an odd quarterwave-length energizer should be current fed as shown in Fig. 2, but that an even quarter-wavelength energizer should be voltage fed when power is furnished at the end of the said energizer. The functional relations between the length of the energizer and its current and voltage distributions, radiation resistance, directivity and effectiveness as a radiator are consistent with established principles of radio engineering. Moreover, the said energizer may be buried beneath the air-earth interface 19, but when this mode of operation is used the energizer ordinarily is oriented substantially parallel to the said interface. Furthermore, the said energizer may be arranged in two (or more) parts with its associated transmitter located centrally thereto, and one (or more) of the said parts may be used to replace the ground rod ll, so that there is no metallic connection with the earth 5. An alternative arrangement of this kind is shown in Fig. 7, where the two similar conductors 10 are each quarter-wave (or other suitable length) energizers, placed flat on the ground surface I9, or buried within the earth 6 as indicated diagrammatically by the dashed lines 26. And finally, it will be understood that various forms of wellknown coupling devices may be employed to connect the said energizer or energizers to the associated transmitter.

The fundamental principle involved in the invention is the provision of a radiating element or elements placed substantially in or below the air-earth boundary, and those skilled in the art will recognize that a multitude of variations may be used in practicing the invention. The quarterwave-length horizontal insulated energizer [0 (Fig. 2), located on the ground surface I9, is the preferred means because it accomplishes the desired purpose with simple and effective apparatus, which is convenient to handle in practical field operations.

It will be understood that the length of the preferred quarter-wave energizer refers to the wave length in the earth of the propagated waves, and that the earth wave length is equal to the wave length in free space divided by the refractive index of the earth material in question with respect to air. The wave length in feet in free space is equal to 9.85 l0 divided by the frequency in cycles per second, and for ordinary earth the index of refraction generally runs between about 1.25 and 3.00, which means that the respective velocity of the waves in the earth usually is between 0.80 and 0.33 that in free space. A quarter-wave energizer is one which has an electrical length approximating one-quarter the length of the radiated waves in the earth, it being remembered that the true length of the energizer Ill (Fig. 2) should be measured from its free end 52 to the ground connection ll, taking into account the fact that the condenser M has the effect of decreasing the electrical length of the said energizer, and that the inductor [5 has the efiect of increasing the electrical length of the said energizer.

The electrical length of a given energizer, positioned on earth having particular electrical properties, may be determined experimentally in various well-known Ways, for instance, by finding its current loops and nodes with an exploring coil connected to a sensitive current-indicating meter, or by locating its voltage loops and nodes InC.,'NeW"Y0rk(1933?? v As coincident current 'nodesfiand yoltage loops, and coincident current'=-1oons and*voltage nodes-, willbe found at intervals'of one-half wave-rescue, along the energizer; it willbe "seen tha't theme I ferr'ed quarter waveenergizermay b obtainect io i of-the'refle'ctecl waves (for example, the Wave by varying the length of theconductor l fl until' 1 a currentuo-op and; voltage'nodappear atthe=-= ground connection l'l anii a current-"node an voltage loop are presentat the fr'eaerid- 'liz of th saidconductor. The adj ustment for' eng-tli o the" energizer-preferably 'is madewit 'the con denserl l setnearits'midscale posi-t'i'o sub stantially canceling the reactancefofthe coilso thatthe length so-determiried be'used' in various localitiesi havin'g diiferent types 0 I without exceeding the tuning rangaofthesaid condenser. g

The specification thusfarhas' di'rected atten-v tion to means foramia methodf-of-efiicie t1 I loop. All rays thatemerge to-the-right or left of the=said=loopremain= undetected; For the case l1ere ci-ted, it is; accordingly, only a very small part of the emergent energy that is effective in the operation-of a receiving means which is energized-by-auy type of portable antenna that is placed in the air 33, above the interface 19.

Furthermore, if the reflector 29 be disposed as illustrated in 5, then the angle of incidence represented by the ray in) on the air-earth interface 1 Qtvillexceerl the critical angle,i =sinl/n,

and the said Waves will be totally reflected at the said interface, as indicated by the ray In'this 15 case there can be no emergent Waves, and hence a receiving means that is excited by any'forin. of

antennaplaced'in the air 33, a substantialdistanceabove the'inter-face :ae, willnot prove effective indetecting the energyreturned to the 1th 2o ground surface by the reflector 29.

The three cases here referred to cover broadly,

the principal applications of the radio 'methods, and it is'seen that the conventional receiving means are adapted to the solution of but one of.

transmitting electromagnetic*energyfpastthealr= the cases, namely, that which i voives a reflector quently is returned to the air' earth" interfacer- 3G The effective reception of the returned energy" is, therefore,- essential forthe propen recognition'of the subsurface conditionsinvolved-.- It will be understood from what -folloivsthat thecon ventiona-l receiving meansusedwith -the presently disclosed "radio -pro'spectin'g "methods "are" subjcct to serious limitations'iin' their 'reception'oi" electromagnetic 'wvaves that emerge from; the J earth.

or reflectors lying suhstantialiy parallel .to'the air-earthinterface', and the emergence of -Waves that are refracte'rl substantially parallel to the said interface 7 7 it will'be understood'from'what follows that the herein disclosedv invention'may. be efie'ctively ap-' plied not only in 'the'transmission' of electromag netic waves into the earth',but1'a1so'in theirre' ception therefrom; and when u so applied; the -in vention makes p'ossible the detection of all waves arriving from depth "at the air earth interface,

regardless of their angles" ofinci'clence' thereon. In Fig. 6 isillnstrated'a inetallic conductor Gd,

covererl -byi the insulating sheath which is Line 21 'of"Fig-.;3 representsraeroup'of electro'-" placed fiat onthe ground surface i e fan'dsconmagnetic waves'propagatedby a'suitablezmecha nismg'forillustration; the energizer I 0 "showfi'dia grammatically in Fig. 2.. The waves" m'ove-rrom' leftto right and strike'at'substantially razing.

incidence the air eartli intflerfa'ce' I QjTIWhereItheyL water, therrthey .will bereflected asindicated by rineien eviromthe"s i "*i 1' m rv t -"the said the ray 39 to the said air=eartli interface 15, and" thence-refracted as shownbythe ray'iii l Radio prospecting system's customarily em'hbdifa ire ceivin'g'means thatisexoited by'airantenriaf rer example; theloopantenna; 32;whi'chfls located m:

nected through the variable condenser it? to one end of the primary er or a radio frequency trans-' former, the secondary ii: of which is tied to the input of the receiver R), 'enclosed by the electro v magnetic shield 53. The electrostatic 'shie1d5, which is located between the primary "and secondary "-liil' of th'e said traii'sfdrmer and con-- denser 25,1 a'ra'dio 'wavei'rnoving in eith'endirection along-the sai'i conductor Will cause 'a 0111'- rent to iio'w therein, and the said cu rent may be J fed into any suitable receiving means; such as the receiver- R; vvhoseoutput-- be i-passezl w through an: amplifier. andindicated .or measured p by a-meter-or other suitable nieansw As will-be shown hereinafter; the conductor i i-has geod cli-. rective discrimination; s its receptiveness being greatest :for a=wave moving along its length and least' fona wave -movingcat right angles thereto.

Positioned" substantially? in (or below) the air-= eartlrinterface-i 9; the-said conductorissuitable for thewre'ceptio'n' of Waves" traveling parallel to the isaid sinterfaceyas illustrated in Fig and as showrrin Fig: lfand.theen'iergenifraysarenot"" waves t'hat arrive 'fronr'deptli and'striize' thefsai'cl fectivefin"causingacurrentto ilo'w irithlrlbp V electromagnetic waves that arrive att' e-air-earth" are thoserayswhicNhappento strike the saith- 5' infi f 9 frbm riydirectionl andbecauseof its interface at variousfan'glesiof incidence,lsuch'.as mdicated in -Figs' -e "and 5; The conductor i l- "tner'erore performs tnerunetien 'of" king up particular function the said conductor shall be referred to at times in what follows as a pickup.

The intensity of the signal reaching the receiver R generally will be increased if the shield 50 is connected to the earth :5 by a ground rod driven into the said earth, but in practice this is an inconvenience which usually is unnecessary, and for ordinary operations it will be found that satisfactory results may be obtained with the receiver R, and its associated shield 59, placed on the ground surface 59, or even elevated an appreciable distance therefrom.

The receptiveness of the pickup it also varies with its length, becoming progressively greater up to an electrical length equal to one-quarter the length (in earth) of the received waves. Lengths in excess of one-quarter wave usually are difiicult to handle in field operations; result in little improvement over the performance of the quarter-wave pickup, and require no further consideration here.

The preferred means for using the herein invention for the reception of electromagnetic waves depends somewhat on the wave length of the radiation involved, on the power radiated by the associated transmitter, and on the sensitivity of the receiver. It has been found satisfactory in practice to employ a rubber-covered single-conductor stranded cable for the pickup dd of Fig. 6, and to make the length of the said cable approximately one-quarter the length (in earth) of the received waves. Should this lead to a length that is difficult to use in the field, then it usually can be reduced to a convenient size without seriously impairing the effectiveness of the invention. Aside from differences in the coupling means ordinarily employed, it is seen that the preferred form of pickup (Fig. 5) is quite similar in its es sential characteristics to the preferred form of energizer (Fig. 2). With the latter, however, it is usually desirable to provide more effective insulation about the conducting element, particularly near its free end 52, where high voltage may be present.

It will be evident to those familiar with radio practice that alternative arrangements of the receiving means may at times be employed to ad vantage. For instance, with some kinds of earth material the pickup id may consist of a bare wire, of various sizes and lengths, which makes intimate electrical contact with the adjacent earth 6. Moreover, the said pickup may be buried in the said earth below the ground surface id, and ordinarily parallel thereto, instead of being placed above the said surface as indicated in Fig. 6. ternatively, the receiving means may be a double ended system as illustrated in Fig. 8, which comprises two (or more) similar quarter-wave-length (or other suitable length) conductors extending in substantially opposite directions from the receiver R, and the said conductors may be located on the ground surface it, or they may be buried in the earth 6 as shown diagrammatically by the dashed lines 53.

Various other modifications will occur to those skilled in the radio art without deviating from the basic principle involved in the herein inventicn, namely, the provision of a receiving element or elements placed substantially in or below the air-earth interface, so as to furnish an effective means for the detection of electromagnetic waves that arrive from depth at the said interface, irrespective of the paths traveled by the said waves.

One end of the buried energizers 26 of Fig. 7 and one end of the buried pickups $3 of Fig. 8

are brought above the earths surface l9 for connection with the respective transmitting means T and receiving means R. This is done ordinarily as a matter of convenience in practice, and for the sake of simplification in the drawings. It should be understood, however, that an energizer or pickup may be operated when completely enclosed within the earth, with its associated transmitting means or receiving means likewise buried, or connected by suitable coupling devices and transmission lines to transmitting means or receiving means positioned on or above the earths surface. The lastunentioned procedure is, of course, the preferable arrangement in the case of a deeply buried energizer or pickup, whether a single-ended or double-ended system is used.

It should not be inferred that the energizer or pickup disclosed herein must necessarily embody a linear conductor or conductors as illustrated by the accompanying drawings, for it is evident that other forms of transmitting and receiving elements may be used in practicing the invention; for instance, the said transmitting and receiving elements may comprise a conductor (or conductors) wound in the form of a loop (or loops), and the said loop may be used in accordance with the herein disclosures if it be placed substantially in or below the air-earth interface, with its plane preferably parallel to the direction of the outgoing or incoming waves.

Preferred forms of the energizer and pickup now have been described, and alternative means for practicing the invention have been suggested. In each case, however, it has been indicated that in the preferred mode of operation the said energizer and pickup are placed substantially parallel to the air-earth interface, so that the propagated energy is transmitted into the earth and received therefrom principally at grazing incidence.

One should recognize, nevertheless, that the said energizer and pickup may be oriented in the earth in directions other than parallel to its surface, and when so oriented they provide a means for directing electromagnetic energy into the earth, and receiving it therefrom, in directions other than substantially parallel to the air-earth interface.

For example, the radiation from an energizer may be directed into the earth in various and arbitrary directions by the simple expedient of positioning the said energizer in a bore hole (or other suitable opening) that is drilled or otherwise formed in the said earth at the proper angle therein.

It is obvious that the same principle may be applied with equal facility to the pickup described in this specification, and when so applied the invention provides a means for the reception of electromagnetic energy returning in various di rections from depth in the earth.

The combination of a buried energizer and a buried pickup, whose mutual spatial relation can be chosen to meet particular requirements, may sometimes be used advantageously in the investigation of certain subsurface problems where it is desirable to emphasize the reception of waves from a particular reflector lying at a particular depth, and thereby discriminate against the reflections that arise from shallower or deeper refiectors. This procedure clearly will simplify the character of the response indicated by the receiving means, and make more reliable an interpretation of the subsurface conditions postulated on the said response.

The energizer-pickup combination here de- '11 "scribed may be incorporated-in theapparatus emzbodied in yarious systems for exploring-the geologic section with electromagneticwaves, for instance, in the system set; forth irrapplicants j copending application Serial -No. 383,7?9," filed march 17, 1941; nOWPateI-ItN012,573',682;entitled Means and Methods for Electromagne'tic -Wave Tlnvestigations. When "used with applicants "saidsystem, itgis' to be understood that the lengths "of the energizer; and pickupv are varied-in acco'rd-' Although thisfeature is sometimes an advantage for certain applications,-yet in other-cases it is "preferable that the radiation be unidirectional-,so that the energy is concentrated in one-or the other direction.

The preferred "method of obtaining unidirecergizer 55, placed flat' on the earth's surface and grounded at thefi'point' t. -For sim'plicity, the tuning components, resonance-indicating means and coupling device; which usually are included in the energizer circuit, have been omitted in-the ydrawing. Worth noting at this point is the fact that the horizontal reception pattern of a single quarter-wave insulated pickup, located flat on-the earths surface, hasessentially the same configuration as the radiation-pattern 5d of- Fig. 9.

Now, if two ,collinearyquarter waveinsulated *energizers' 5i and58 be-disposed on the earths surface as shown in Figflfl, withtheir ground Qconnections 59 and 59'separated substantially a "quarter-Wave length (inearth), the unidirectional horizontal; radiation pattern 6 l-'will"be;developedwhen the current supplied-the energizer 57 leads the current in the ,energizer -M-byQO degrees. The energizer 5'i't1hus'acts as a driven reflector, which 'cutsoutthe lobetto the leftof Fig. 9, and makes the combinationsubstantially unidirectional. If the'phase of the current in 53 were 90 degrees ahead of. the current in- 57,then the two collinear energizers would remain unidirectional, but would fire in theopposite direction. In this case, as with othermultiple-element energizer arrays, each element may be fed by a low-impedance transmission line which derives its power from a conventional type of phaseshifting network. 'One such arrangementis illustrated diagrammaticallyin)Fig. '12, where-the transmitter T supplies r ra'dio frequency power through the phase-'oontr'oldevice. 69 a-nd' thence throughi the coaxialcables"ifl andll to'the like --coup'ling and amplitude-control device12," which feed the energizersffil and 58. "C'Ihe cou'plingand :amplitude-control devices 'EZ'may be of the" type :shown .in Fig.32, .wherein 'the coilsfl5 and 18 are in the form of a variometer, whichprovides a convenient means for: varying the-coupling between the coils and consequently, the amplitudes of the currents supplied'the respective energizers. The radiation pattern of Fig. 10 can be made more directional by the addition of a driven director to the right ofthe elementEB, or preferably, by adding -broadside'pairs of collinear astures quarter wave -energizers;--such as the three simi- *lar pairs illustrated inFig. 1l',"where' the spacing -between pairs-is made substantially'one-half wave length. The unidirectional horizontal lobe 62 is 5* obtained when theener'gizers' 63, E34 and 65 are 1 furnishedcurrents of the same'phase, but 9% degreesahead of'the respectively like currents :in the collinear energizers 5i and 53, and the currents in the center pair of energizers are made -l0'*twice-the magnitude of those in the outside pairs.

The directivity of the array may be increased by the addition of broadside elements. "The six- I element array of Fig. 11 may be driven inaccord an'ce" with'the diagrammatic layout of Fig. '13, 15-where the energizers are numbered as inFigfll =.and theremaining components represent the'likcnumberedcomponents of-Fig. 12. With the ar- "rangem'ent of Fig. 13, it may be preferable to have the lengths of the low-impedance cables 33, and 15 substantially the same, in order that currents of like phase be delivered to the phase --=control--devices' 69. These devices mayif desired, *berelolaced by phase-delay cables, so that the-currents in the forward elements in Figs. 12 and 13 lag-90 degrees behind the reflector currents.

-lit should be stated here that the radiation patterns of Figs. 9, l0 and 11 are based on'the assump-tion that equal amounts of power aresupplied in 'eachcase to the energizer system, regardlcss of the number of'elements embodied in the said system.

From the foregoing it will be evident to one familiar with directive arrays thatunidirectional radiation patterns of substantially any configuration can be secured by various combinations of energizers, whose arrangement, number, lengths, "spacing, phasing and currents, as well as their types (insulated or uninsulated) and depth of burial, are chosen'properly'to meet the-require- 40 ments' of the'problem. 'A proper choice of the variableshere-set forth also makes it'possible to change 'thedirectionof maximum propagation of *the "energizer system.

Owing to .thereciprocity relation that exists between theradiating and receiving characteristics 'of directive arrays, it will be found that pickupsjmaybe used in the arrays hereinabove "discussedinstead of energizers, so as to greatly emphasize the reception of waves that arrive 'from aparticular direction. It will also be found that the reception pattern of a given array of pickups will be substantially the same as the radiation pattern of the same array of energicers,

and that the pickup array and energizer array Will respectively receive and radiate most efiectively along the same wave path.

Earlier in this specification a mode of operation was described which involved the positioning of an energizer and/ or a pick-up at arbitrary angles-within the earth, so as to control the direction 'of propagation and/or reception of waves 'therewithin. With further reference to thismode of operation, it willbe clear that a unidirectional array of energizers may be substituted for-the single energizer specified; that a unidirectional array of pickups may be used to replace the single pickupmentioned previously, and thatithe com- "bination of the energizer array and the pickup array will greatly improve the directive charac- =te'ris'tics and functioning of the system discussed. When considering the application here referred to, one should remember that the direction of maxi- -mum propagation of an energizer array, and the direction of maximum reception of a pickup array,

. 5 may be varied iIr-ways other than by changing the orientation of the array in question. For example, with the buried arrays under discussion, the directions of maximum propagation and reception may be varied over a considerable range by a proper phasing of the currents flowing in the array elements. This can often be of practical significance with buried arrays, whose radiating and receiving characteristics may be adjusted to meet particular operating conditions without changing the elements or their positions in the earth.

It will be clear than an energizer system may comprise multiple elements, some of which are positioned above those lying substantially in or below the air-earth interface, and that a combination of this kind makes it possible to vary in a vertical plane the radiation pattern and the direction of maximum wave propagation, thereby providing an alternative means for securing vertical directivity without the necessity of placing the energizer array in bore holes (or other openings) in the earth. A combination of the type here referred to may comprise one or more radiating elements and one or more parasitic or driven reflecting and/or directing elements, and the said combination may be used not only for the directional transmission of electromagnetic waves into the earth, but also for the directional reception of waves returning to the surface from depth in the earth. The theory, design and construction of directive transmitting and receiving systerns are treated comprehensively in many textbooks and other publications, and therefore it is unnecessary to consider here the detailed application of such systems in terms of the herein invention, other than to state that either the entire energizer or pickup structure is placed within the earth, or not less than the lowermost element or elements of the said structure are positioned substantially in or below the ground surface which constitutes the air-earth interface.

When parasitic reflectors and/or directors are employed with any of the energizer arrays referred to herein, it is to be remembered that the said arrays can be made to function when only one of the radiating elements is excited by a power source, since the reflectors and directors will be energized by wave excitation. And in like manner, a pickup array embodying parasitic reflectors and/or directors can be operated when I only one of its receiving elements is connected to a receiving means.

It should be emphasized here that while the preferred transmitting and receiving means disclosed in the specification and drawings make use of resonant elements placed substantially in or below the air-earth interface, nevertheless, the said means will also function if operated in a nonresonant condition. The effectiveness of the said means is greatly improved, however, when the said elements are made resonant to the frequency of the electromagnetic energy involved.

The preferred form of radiating means shown in Fig. 2 propagates vertically polarized waves, which is the most efiective polarization for transmission into the earth, and the preferred form of receiving means illustrated in Fig. 6 receives either vertically or horizontally polarized waves, or, in fact, waves that are polarized in any direction. This feature of the said receiving means adapts it to the requirements of almost any radioprospecting method, whether or not it be employed in conjunction with the transmitting means disclosed herein. Furthermore, the said transmitting means may be used with other types of receiving means, provided they are adapted to the reception of vertically polarized waves. The transmitting and receiving means described in this specification may therefore be incorporated individually with other radio-prospecting apparatus, or they may be used collectively in practicing various radio methods of exploration.

For example, an energizer, or an appropriate combination of energizers, may be employed to propagate the electromagnetic energy delivered by the transmitter embodied in the geophysical system disclosed in applicants U. S. Patent No. 2,172,688, issued September 12, 1939, under the title Electrical Apparatus and Method for Geologic Studies, and the said energy may be received with the loop antenna described in the aforesaid patent. Moreover, the pickup disclosed herein, or a combination thereof, may be used instead of the said loop antenna for receiving the waves radiated by the air antenna disclosed in the said patent. Alternatively, the said air antenna and loop antenna may both be replaced by the combination of an energizer and pickup, which thereby provide an effective means for practicing the method set forth in the said patent.

From what has gone before it will be readily apparent that when the transmitting means and the receiving means disclosed herein are used in conjunction with one another, they form a complete and improved system for effectively propagating electromagnetic energy into the earth and receiving it therefrom. As this is the fundamental requirement of most radio-prospecting techniques, it will be understood that the said system may be used advantageously in practicing a large number of the radio methods described in the art and literature.

In the preceding discussion, and in the claims to follow, it is indicated that electromagnetic energy is transmitted from an energizer in the form of electromagnetic waves, or more restrictedly as radio waves (unmodulated or modulated, continuous or interrupted), and that the said energy is received in the same form by a pickup. It is not intended, however, to limit the operation of the said energizer or pickup to the respective transmission and reception of electromagnetic waves, or radio waves, for it is recognized that the said operation may be postulated on the use of other forms of electric-energy flow, for example, on electric currents and pulses. Wave transmission and reception should not therefore be regarded as the only operative mechanism, but rather the preferred mechanism.

t is to be understood that the apparatus and methods disclosed herein are susceptible of various modifications without departing from the spirit and broad principles of the invention, and accordingly it is desired to claim all novelty inherent in the invention as broadly as the prior art permits.

What is claimed as new and useful is:

1. In a system for the unidirectional transmission of electromagnetic waves into the earth, an array comprising a plurality of collinear pairs of elongated energizers whose l ngths approximate an odd quarter-wave multiple of the wave length in the earth of the eiectromagnetic waves to be transmitted by said energizers, said energizers being placed substantially below and approximately parallel to the ainearth interface and separated approximately onehalf wave length between adjacent pairs, means for tuning said energizers to the frequency of said waves,

15 said-.tumngameansbeine adapted to warr n -the electrical lengths ,;:of :isaideenergizers without modifying their fixed physical ,lengths,meansz for geq-ualizing the currents flowing iii-each collinear ;pair of said.,energizers-in'ieansyfor coupling saidof.theelectromagnetic waves tube transmitted byasaid energizers, said-energizers being placed adjacent? to one another-and in. collinear relationship' substantially below the. air-earth inter- ,=:face,-and approximately parallel thereto, eachof aesaid energizers consisting of ametallic, core surgsroundedby insulating niaterial separating said core from said earth throughout substantially atheentire contactinglengthof, said core with isaidlearth, an electric circuit including .tuning emeans electrically connected to one of said energizers, another electric circuit including tuning means electrically connected to the other of said :energizers-, means for grounding one endof-each ,of said circuits comprising said energizers, means -for generating said waves, means for transfervringelectromagnetic-wave energy from waveenerating'means to said circuits and said ener- -=gizers electrically connected thereto, and means for making the current in one of said energizers substantially 90 electrical degrees out of phase :with-the current in the otherof said energiaers.

3. In a meansfor the unidirectional transmissionof electromagnetic waves into the earth, a directive array comprisingthere collinear pairs of. substantially quarter-wave energizers which erform two sets of broadside elements that are .placed .substantially below and approximately parallel to the air-earth interface and separated ,substantially one-half wave length between adja- ,.,cent collinear pairs,.means for tuningsaid ener- 1 gizers to the frequency, of the electromagnetic waves to be transmitted therefrom, means for a coupling each of said energizers to atransmitter, -,means for making thecurrents flowing in the out- .side pairs of said energizers substantially equal and substantially one-half the magnitude of the currentsflowing in theicenter pair of said enerigizers, andmmeans formaking the phase of the currents in one set of broadside elements substantiallythe same and substantially 90 electrical degrees out of phase with the currents in the other set of broadside elements.

4. In a system for the unidirectional transmission of electromagnetic waves into the earth, an array comprising a plurality of collinear pairs of .e1ongated energizers whose fixed, lengths approximate an odd quarter-wave; multipleof ,the Wave length in the earth of the electromagnetic waves to be transmittedby said energizers, said energizers being placed substantially in and ap- -'proximately parallel tothe air-earth interface --and separated approximately one-half wave length between adjacent pairs, means for tun- -ing said energizers to the frequency ofsaid waves, said tuning means being adapted to varying the electrical lengths or" said energizers without modifying their fixed physical lengths, means for equalizing the currents flowing in each collinear pair of said energizers, means for coupling said 'energizersto a transmitter, and means for-con- ,troll-B .therelativephase,of the currentsflowin-g "in reach collinearapair, of. said ,energizers.

5. .In a system for ,the unidirectional trans- ,missionioi electromagneticwaves into the earth, 5 is, ,directive'arraycomprising two elongated energizers whose lengths, approximate, odd quarv .terewave multiple vof the wave length in the earthoi the electromagnetic wavesto betrans vinitted by-said .energizersQsaid energizers being .placedladiacent'tonne another and in collinear relationship substantially in the air-earth in- .iteriacc,andapproximately parallel thereto, each of .saidenergiaers .consistingoi.a. ,inetallic core surroundediby, insulating material separating 1 said, core irom. said earth ll'il'iioughout substantiallythe entire contacting lengthoi said core "with said earth, an electric,circuitincludi tuningmeans. electrically connected to one of said .energizers, anotherelectric circuit including tun- ,ingameans-electrically connected to the other ,ofusaidenergizers meansv for grounding one end ,of each ,of,,said,.cir cuits comprising said energizers, means iorgenerating said waves, means for 1 transfer ig electromagneticwavc energy v Qfrom wave-generating means to said circuits and said ,energizers electrically connected thereto, and means for making the current in one of said energizers. substantially 9O electrical degrees out of phase with the current, in the other of said energizers.

6. In a means for the unidirectional transmission of electromagnetic waves into the earth, a directive array comprising three collinear pairs of, substantially quarter-wave energizers which form twosets of broadside e1ements that are placedsubstantially .in and approximately par- .allel to the air-earth interface/and separated substantially one-halfwave length between adjacent collinear pairs, means for tuning said ener- 44) gizers to the, frequency of the electromagnetic waves to be transmitted'therefrom, means for couplingleach of ,saidenergizers to a transmitter, means for making thecurrentsflowing in the outside pairsof said energizers substantially equal andsubstantially one-half the magnitude of the currents flowing in thecenter pair of said energizers, and means for making the phase of the currents in one set of broadside elements substantially the same and substantially as electrical K degrees, out of vphase with the currents in the other set of broadsideelements.

7. A method of exploring the subsurface with electrical energy,,comprising generating electro- ,magnetic waveswith a transmitter electrically coupledto an array of energizers positioned substantially bclowthe air-earth interface, adjusting ,said, array ofenergizers to direct its maximum vdownward radiation, toward a subsurface re rilectorhpositionin an. array. of pickups substan- -,tially below .saidinterfaeeand at a distance from said, array of;.energizers, adjusting said array of pickups for maximum reception of the radiation Pemittedby. Said array of ,energizers. and reflected I .by said reflector, and receiving the waves arriving at ,said array .of pickups with a receiving ,means electrically coupled, thereto.

.,.8..In, asystem for the unidirectional transmmission of electromagnetic waves into the earth, a directive arraycomprising two elongated ener- J gizers whose lengths approximate an odd quarter- .wave multiple of the wave length in the earth of the: electromagnetic waves to be transmitted -bysaidenergizers, said energizers being placed :adjacent ,to, one anotheran-d in collinear rela- 75. -tionship substantially below. ,theair-earthinterface and approximately parallel thereto, each of said energizers consisting of a metallic core in conductive relationship with said earth throughout substantially the entire contacting length of said core with said earth, an electric circuit including tuning means electrically connected to one of said energizers, another electric circuit including tuning means electrically connected to the other of said energizers, means for grounding one end of each of said circuits comprising said energizers, means for generating said waves, means for transferring electromagnetic-wave energy from wave-generating means to said circuits and said energizers electrically connected thereto, and means for making the current in one of said energizers substantially 90 electrical degrees out of phase with the current in the other of said energizers.

9. In a system for the unidirectional transmission of electromagnetic waves into the earth, a directive array comprising two elongated ener gizers whose lengths approximate an odd quarterwave multiple of the wave length in the earth of the electromagnetic waves to be transmitted by said energizers, said energizers being placed adjacent to one another and in collinear relationship substantially in the air-earth interface and approximately parallel thereto, each of said energizers consisting of a metallic core in conductive relationship with said earth throughout substantially the entire contacting length of said core with said earth, an electric circuit including WILLIAM M. BARRET.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,220,005 Rogers et al Mar. 20, 1917 1,303,730 Rogers May 13, 1919 1,530,129 Loftin et al Mar. 17, 1925 1,564,940 Chapman Dec. 8, 1925 1,922,115 Stone Aug. 15, 1933 2,139,460 Potapenko Dec. 6, 1938 2,172,688 Barret Sept. 12, 1939 2,286,839 Schelkunoff 1 June 16, 1942 2,419,562 Kandoian Apr. 29, 1947 OTHER REFERENCES Radio Engineering by Terman, 2nd Edition, 1937, pages 663-671, McGraw-Hill Book Co. 

